1. Technical Field
The present invention pertains to a device and technique for providing a stable optical sensor system. In particular, the invention concerns a variable gain amplifier circuit for controlling leakage current in an optical sensor system of a bill validator, and a method for obtaining light measurements.
2. Background Information
Bill validators used in vending machines and the like typically utilize optical, magnetic and other sensors to obtain measurements from an inserted bill to determine authenticity and denomination. Typically, the bill is transported past a plurality of photosensors, typically made up of light-emitting diodes (LEDs) and phototransistors, and a plurality of signals are generated corresponding to measurements taken from different areas of the bill. The LED drive signals are typically multiplexed between two or more LEDs to emit light at a number of wavelengths in a short period of time to ensure high security against fraud. The signals are then processed to validate and authenticate the bill.
Factors which may adversely affect the bill measurements include the following: the output power of an LED can degrade on the order of fifty percent with aging, inserted bills are of differing quality, and bill path cleanliness may vary. Further, some bill validator applications require undiminished performance under harsh outdoor conditions such as extreme cold or heat, and such temperature changes can cause wildly varying gain characteristics in LED-receiver pairs resulting in erroneous sensor readings. In addition, there are normal production variations in LED optical power output and receiver sensitivity, which can lead to units having varying current and voltage requirements in order to operate effectively. In order to compensate for such phenomena, optical sensor measurements were taken over a large dynamic range. However, a gain adjustment range for the amplifier circuitry of ten to one was typically used, which proved to be marginally adequate only if tightly specified and sorted optical components were utilized in the bill validator. Since performance of a bill validator must be acceptable over a wide temperature range, and LED/receiver pairs are subject to variations in performance characteristics, a system having an adjustable gain is required to ensure a robust bill validator sensor system.